The British government is sending refused asylum seekers back home, a Guardian investigation has revealed, despite the fears of human rights campaigners and lawyers that deportees could encounter persecution on their return.

The government claims that those forcibly returned will be safe.

There are an estimated 10,000 Congolese asylum seekers in the UK, many of whom are at risk of being forcibly removed. The sending back of such people to the Democratic Republic of the Congo was suspended in 2007 but recently resumed.

The revelations about the possible torture in Congo came as the government intensified its operation to forcibly remove Congolese nationals from the UK. Last Thursday there was a charter flight carrying 24 Congolese bound for Kinshasa, the first such flight for more than two years.

Nsimba Kumbi, 33, a refused asylum seeker, was removed from the UK on 13 March, following detention in the Campsfield immigration removal centre in Oxfordshire. He was then detained in the DRC capital, and taken to the notorious secret police headquarters Kin Mazière, the Kinshasa headquarters of the general directorate of intelligence and special services, where, he says, he was tortured for three weeks.

Kumbi says that during his incarceration he was badly beaten, that he received burns and was forced to give a male guard oral sex while his hands were tied behind his back. He says he is now in so much pain he can only move his neck in one direction. The wounds on his back from beatings are gradually drying. He says that nerve damage means he can barely move his fingers.

Another Congolese refused asylum seeker, Rabin Waba Muambi, 42, was also removed from the UK on 13 March on a separate flight. He arrived in Kinshasa the following day and was taken to Kin Mazière. Secret entries in the Kin Mazière log book, leaked to the Guardian, confirm the men's detention there.

Muambi, who says he was beaten at Kinshasa airport, was later stripped, then continually beaten and forced to lie outside staring at the sun.

"The government in Congo views Congolese people deported back home from the UK as enemies," said Muambi. "The guards who beat me kept saying to me: 'You are a spy, give us information, why are you doing this to your country?' I am in a lot of pain from the torture but I'm too scared to go to hospital in case someone recognises me and I get taken back to detention. The guards forced me to drink my own urine and kicked me. They beat me very badly on my knees and made me walk on my knees so they turned black."

He was held there until 14 May when a friend paid a bribe that freed him.

Kumbi's ordeal began in 2001 when he was studying law at the University of Kinshasa. He was arrested for political activities against the government and detained for more than a year in Camp Tshatshi, as place identified by Amnesty International in 2007 as a site where extra-judicial executions of a large number of detainees took place.

He escaped to the UK and claimed asylum in May 2002. Following the rejection of his asylum claim he was detained and removed. He resisted removal and was restrained on the flight. "I was handcuffed and my head was pushed down between my legs. They were beating me for no reason." The escorts handed him over to Congolese immigration officials at the airport in Kinshasa and he was taken to a windowless room. "It was like a store room. The men were wearing military uniforms and they forced me to sign some papers without letting me read them. I had no choice but to sign and hoped that after I signed I'd be released."

Instead he was taken to Kin Mazière. "The guards there demanded money. They said: 'You've just come from England, you must have money.' They said I wouldn't be beaten as much if I gave them money but I had nothing to give them. I was taken to a cell and could hear sounds of screaming and crying coming from the other cells."

Over three weeks at the centre he says he was tortured by the guards. Following his release he went into hiding. He says he is in urgent need of medical treatment but it is too risky to go to a hospital.

Muambi first came to the attention of the Congolese authorities in January 2005 when he was involved in anti-government protests. He was detained and tortured for a month and then escaped to the UK and claimed asylum.

He was diagnosed with post-traumatic stress disorder. He belongs to the human rights group Apareco,whose members risk torture in Congo.

Rudi Ramdarshan, a human rights lawyer at Trott & Gentry solicitors, said: "The implications of these cases are very worrying. The Home Office's own operational guidance states that if people end up in detention in Congo they are likely to experience inhumane and degrading treatment, in breach of article 3 of the Human Rights Act. The fact that asylum seekers returned to Congo are ending up in detention is unacceptable.

"I am calling on Amnesty International and Human Rights Watch to launch investigations. The secretary of state should suspend removals while this matter is investigated."

Last month Amnesty International submitted a report to the UN highlighting the persistence of torture and ill treatment in detention in Congo.

A spokesman for the UK Border Agency confirmed that his department had removed the two men from the UK. "We continue to monitor developments in the Democratic Republic of the Congo and will take decisions on a case-by-case basis in light of the most current situation. The court of appeal on 3 December 2008 upheld a ruling that failed asylum seekers are not at risk of persecution or ill-treatment on return to Democratic Republic of the Congo simply because they have claimed asylum.

"We do not routinely monitor the treatment of individuals once removed from the UK. We would not remove them if we considered them likely to suffer persecution on their return and there is independent oversight by the courts."

The spokesman, referring to Kumbi's claims of being beaten by UK escorts, said: "We are committed to ensuring that removals are carried out in the most sensitive way possible, treating those being removed with courtesy and dignity. Any allegations of misconduct are thoroughly investigated and all allegations of abuse are referred to the police."

The Congolese embassy was unavailable for comment.

• This article was amended on 1 June 2009. In editing of the original piece, a reference was added to the photo caption saying that the picture was covertly taken. This has been corrected.